External hard drive not being recognized

Hey everyone......

I have an external hard drive that I use for backing up my Macbook hard drive using "carbon copy cloner". Every so often I run a complete system copy just in case my hard drive crashes (AGAIN). The last time I connected the external hard drive, it wasn't recognized. I don't know where to begin and figure out the problem. Up to now, it's been working great.

I've tried unplugging it, turning it off and on etc.......

Any suggestions?

have you tried restarting the computer without the harddrive connected?

Yes, tried that. But, now it's working???????